remove unused friend class monitor

src/xtd.core/include/xtd/threading/thread.h

@@ -19,7 +19,6 @@ namespace xtd {
   /// @brief The xtd::threading namespace provides classes and interfaces that enable multithreaded programming. In addition to classes for synchronizing thread activities and access to data ( xtd::threading::mutex, xtd::threading::monitor, xtd::threading::interlocked, xtd::threading::auto_reset_event, and so on), this namespace includes a xtd::threading::thread_pool class that allows you to use a pool of system-supplied threads, and a xtd::threading::timer class that executes callback methods on thread pool threads.
   namespace threading {
     /// @cond
-    class monitor;
     class thread_pool;
     /// @endcond
 
@@ -374,7 +373,6 @@ namespace xtd {
       /// @endcond
 
     private:
-      friend class monitor;
       friend class thread_pool;
       friend class wait_handle;